As is known, tire tread strips comprise sculpted patterns on their radially external surface, with a view to improving the road_holding of the tires on surfaces of different natures. Thus, in general, the tread pattern has grooves directed in the circumferential direction, transverse grooves, and blocks of tread patterns comprising cuts. One of the purposes of these cuts is to provide the tire with better grip on wet ground.
The array of cuts is obtained by arranging a corresponding array of siping blades in the curing mould. These cuts, of a given depth, known as external or open-ended cuts, are produced during the moulding operation and extend radially from the radially external surface of the tread strip towards the inside of the tread strip.
However, it is found that the effectiveness of the external cuts decreases as the tire tread blocks become worn, because of the relative reduction in depth of the said external cuts.
In addition it has been proposed that cuts be made that extend radially over a given height from the radially internal face of the tread strip and which are arranged in line with the blocks of rubber. The height of these internal cuts is determined in such a way that the said internal or blind cuts do not communicate with the radially external face of the tread strip when the tire is not worn. When the blocks have reached a certain level of wear, the external cuts appear at the surface of the tread strip in order to give the tire additional grip until such time as the tread strip is fully worn.
By way of example, publication EP 335 694 describes a tread strip comprising internal cuts made radially from the internal surface of the tread strip.
Numerous publications cover how to produce these internal cuts.
Publication JP 11 147 403 proposes the use of inserts made of a material that has a resistance to wear inferior to that of the tread strip and which, following partial wear, opens onto the radially external surface of the tread strip.
Publication JP 04 113 905 proposes making the internal cuts in a pre-cured tread strip. The tread strip thus modified is then applied to the radially external surface of the tire, generally following a re-treading operation.
Publication DE 10 2006 036 509 describes a method in which the internal cuts are made in an unvulcanized tread strip. A non-stick material is introduced into the cuts to prevent the said cuts from closing up during the vulcanizing operation.
Thus, the problems that arise when internal cuts are to be made in an unvulcanized tread strip and the latter is then to be applied to the green tire during the final phase of tire building, are of two kinds: a first problem, as was seen above, is that of preventing the faces of the cuts from welding themselves back together again during tire vulcanization. The second problem is in the positioning of the internal cuts such that they lie in line with the blocks of the tread pattern, because steps are taken to avoid the internal cuts opening into the bottoms of the circumferential or transverse grooves.